Asymmetric membranes—in use for many years—are characterized by having the pore size of the membrane vary as a function of location within the thickness of the membrane. The most common asymmetric membrane has a gradient structure, in which pore size gradually and continually increases from one surface (often referred to as the “tight” side) to the other (often referred to as the “open” side). These membranes are valued as they have a higher flux than comparable symmetric membranes. When used in the configuration with their larger pore side upstream, these membranes have greater throughput in many cases as compared to the comparable symmetric membranes. See, U.S. Pat. No. 4,261,834, issued to D. M. de Winter on Apr. 14, 1981. The asymmetrical membranes are used in a variety of applications such as food and beverage filtration, pharmaceutical and biopharmaceutical manufacture, laboratory filtration, water filtration and the like. Asymmetrical membranes based on aromatic sulphones such as polyethersulphones are known and are capable of use at elevated temperatures and highly acidic and basic conditions.
Asymmetric membranes all have a thick, dense surface region, or in many cases a skin formed on one surface and extending somewhat into its depth, e.g. a tight surface. See, U.S. Pat. No. 4,629,563, issued to W. Wrasidlo on Dec. 16, 1986. The dense surface and/or skin can be seen through the use of photomicrographs. The dense surface is shown as a continuous dense film surface punctuated by a myriad of pores. The skin can be seen in cross-sectional photomicrographs as a dense layer extending into the thickness of the membrane. See, U.S. Pat. No. 4,629,563.
More recently, a multiple layered asymmetric membrane has been produced. See PCT International Publication No. WO 01/89673. This membrane is formed of two or more layers that are co-cast simultaneously from two or more different solutions of membrane precursor material. Unique asymmetrical structures can be formed with the membranes of this invention.
While most asymmetric membranes work satisfactorily on water or aqueous based solutions, they tend to prematurely clog and have poor throughput with viscous or heavily loaded streams, even when used in the preferred open side upstream configuration. Such streams are relatively common and can include various food streams such as syrups and sugary products, serum streams as used in the biopharmaceutical or laboratory settings or blood, plasma and other blood products.
In light of the above, there is a need for a membrane having high throughput and flux, suitable for quickly filtering high volumes of fluid, e.g. viscous fluids.